For the week, just over 18% of specimens sent to clinical laboratories tested positive for influenza. This is down from 22.5% positive for the previous week. Influenza A (H1) remains the predominant strain reported, but the proportion of H3 reports is increasing. Looking at reports from recent weeks, all 10 regions are now H3 predominant. Region 4 is the only region that has been H3 predominant for the entire season, but Regions 6 and 7 now have about equal numbers of H1s and H3s reported. The amount of reported influenza B circulating viruses continues to remain very low.

During this season, more than 1,700 viruses have been genetically or antigenically characterized. As reported in previous weeks, all of the H1 viruses are the 6B1 clade, and 97% of the antigenically characterized H1 viruses were similar to the cell-propagated vaccine reference virus. Multiple clades and subclades of the H3 virus are co-circulating. At 67%, the 3C3a subclade is by far the predominant subclade in the U.S. this season. Sixty-seven percent (67%) of the H3s are antigenically similar to the cell-grown reference virus representing the vaccine strain. Most of the H3s that had low similarity to the cell-grown reference virus belong to clade 3C3a.

All of the Influenza B Yamagata viruses tested have belonged to the Y3 clade and are antigenically similar to the cell-grown vaccine virus. For the B Victoria viruses, 61% are the V1A.1 clade, which is the same clade as the vaccine. In addition, 76% of the Victoria viruses are antigenically similar to the cell-grown vaccine reference virus. Over 1,700 viruses have been tested for resistance to oseltamivir, peramivir, and zanamivir. A new report for 1 H1 virus specimen shows reduced or highly reduced susceptibility to oseltamivir and/or peramivir. A total of 5 H1 specimens have now been found to have reduced or highly reduced inhibition by oseltamivir and peramivir. However, more than 99% of tested viruses have been susceptible to antiviral medications.

Outpatient visits for influenza-like illness (ILI) declined to 3.2%, down from 3.8% the previous week. This is the sixth week in a row in which a decline has been experienced. Despite this decline, ILI activity is still elevated and has been at or above baseline nationally for 19 consecutive weeks. All 10 surveillance regions are above their region-specific baselines. On a state level, 6 states experienced high ILI levels, 19 states reported moderate activity, 13 states and New York City, the District of Columbia, and Puerto Rico were at low activity, and 12 states reported minimal activity.

Cumulative reports to FluSurv-NET this season indicate a rate of 56.4/100,000 lab-confirmed influenza hospitalizations. Highest hospitalization rates were seen in persons >65 years at almost 182/100,000, followed by adults age 50–64 years at a rate of 72/100,000 and children 0–4 years at approximately 66/100,000. The percent of deaths reported due to pneumonia and influenza (P&I) was 7.2% during the week ending March 23, equal to the epidemic threshold for this week. P&I has been relatively low for the entire season.

Eighty-two (82) influenza-associated pediatric deaths were reported to CDC this season. Of these 76 were associated with influenza A virus infections, 5 with an influenza B virus infection, and 1 with an influenza virus that wasn’t typed. Seventy-six percent (76%) of the A viruses with subtype information available were H1, and 24% were H3.

For the geographic spread of influenza during the week, 33 states and Puerto Rico reported widespread activity. Fifteen (15) states reported regional activity, and 1 state and the District of Columbia reported low activity.

For the preliminary influenza burden reports released, an estimated 33.2 million persons have been ill with flu, and approximately 452,000 were hospitalized. An estimated 30,600 flu-related deaths have occurred.

Other Items – L.J Tan (IAC)

2019 Summit In-Person Meeting Information Available Online – The 2019 Summit in-person meeting will be held in Atlanta, GA on May 14–16, 2019. Information on registration is available on the 2019 National Adult and Influenza Immunization Summit webpage. (Please note that the password to register for this invitation-only meeting is available by contacting L.J Tan.) If possible, please book your room either through the onscreen “pop-up” that appears after you register or later through the email you receive after completing your registration. This not only will give you access to the “preferred rate,” it also will help the Summit meet its lodging contractual agreement with the hotel.